2nd mistake was packing too much stuff. Fixed a week into my trip in a Post office in San Sebastian by stuffing 2/3 of my backpack into a cardboard parcel and posting it home.

My packing rule, slightly tongue in cheek, but not really:

Pack light and then throw out half.

Seriously, I just came back from 3 1/2 weeks in Asia with a backpack with a weight of 11KGs (15, when I returned). I was never short on anything.

This may be slightly different when you have a weighty hobby (scuba diving, or so), kids or go to colder places. But in general my experience is that most people pack far too much.

It's surprising how little you actually need, and as you point out, how little more you need for a 3 week trip than you do for an overnight trip. I don't backpack much but my bikepacking checklist is pretty much the same regardless of the length of the trip. The main (and heavy) variable is the amount of water to bring, and that's more dependent on weather and available water sources than on length of the trip.

Not strictly backpacking, but I went on a car-free trip to Italy a few years back and thought it would be a great idea to buy new shoes for the occasion. After two weeks of walking eight hours a day, the blisters finally healed.